Conventionally, an exhaust gas purification apparatus provided for an exhaust gas passage of an internal combustion engine has been developed, in which a selective catalytic reduction NOx catalyst (hereinafter referred to as “SCR catalyst”) is carried on a filter (see, for example, Patent Document 1). The filter traps the particulate matter (hereinafter referred to as “PM”) contained in the exhaust gas. The SCR catalyst reduces NOx contained in the exhaust gas by using ammonia (NH3) as a reducing agent. The filter, which carries the SCR catalyst as described above, is hereinafter referred to as “SCRF” in some cases.
When SCRF is adopted as the exhaust gas purification apparatus, it is possible to further decrease the size of the exhaust gas purification apparatus as compared with a case in which the filter and the SCR catalyst are separately provided for the exhaust gas passage. Therefore, it is possible to improve the installation performance of the exhaust gas purification apparatus. Further, when SCRF is adopted, the SCR catalyst can be arranged on the more upstream side in the exhaust gas passage. The more upstream the SCR catalyst is arranged in the exhaust gas passage, the more easily the SCR catalyst is heated by the heat of the exhaust gas. Therefore, it is possible to improve the warming-up performance of the SCR catalyst, and it is possible to improve the NOx purification rate of the SCR catalyst.
In this arrangement, trapped PM is accumulated on SCRF. Therefore, the filter regeneration process is executed in an exhaust gas purification system provided with SCRF. The filter regeneration process is the process in which PM accumulated on SCRF is removed by oxidizing PM. The filter regeneration process is realized by supplying the fuel to a pre-catalyst which has the oxidizing function and which is provided for the exhaust gas passage on the downstream side from SCRF. When the fuel is oxidized by the pre-catalyst, the exhaust gas, which flows into SCRF, is heated by the heat of combustion (heat of oxidation). Therefore, the temperature of SCRF can be raised to the filter regeneration temperature at which the oxidation of PM is facilitated.